Centrifugal compressors used in a compressor part or the like of turbochargers in vehicles or ships give a kinetic energy to a fluid through rotation of a vaned wheel and discharge the fluid radially outward to raise the fluid pressure by the centrifugal force.
Various improvements have been made to the scroll structure of the centrifugal compressor in response to the demands for a high pressure ratio and high efficiency in a wide operation range.
Patent Document 1 (Japanese Patent Publication No. 4492045), for example, shows one example of conventional techniques, represented by a centrifugal compressor having a casing with a spirally formed scroll passage. The width in the axial direction of the scroll passage is gradually increased radially from inside to outside, the width being maximum on the radially outer side than the radial midpoint of the passage width.
Patent Document 2 (Japanese Translation of PCT Application No. 2010-529358) relates to a centrifugal compressor for a turbocharger, the compressor having a spiral housing and a diffuser. Patent Document 2 shows that the diffuser is increased in its diameter so as to reduce the low-pressure region in a transition area of the spiral housing (scroll) or an area where the tongue exists.
Patent Document 1: Japanese Patent Publication No. 4492045
Patent Document 2: Japanese Translation of PCT Application No. 2010-529358
As shown in FIGS. 11 and 12, the scroll 13 generally has a circular cross-sectional shape as shown in FIG. 12, and the scroll start and the scroll end of the scroll 13 are connected to form a flow passage joint 04 in a tongue portion 05 shown in FIG. 11.
FIG. 11 shows a front view of a scroll compressor. FIG. 12 shows cross-sectional shapes of the scroll at angles θ1, θ2, . . . advanced by a predetermined amount Δθ clockwise from the tongue portion 05 in an overlaid representation.
In the tongue portion 05, the flow passage joint 04 has a cross-sectional shape with a circular part 09 and a diffuser part 011 contacting this circular part 09, as indicated by the hatches in FIG. 12.
Referring to FIG. 13 which shows the circumferential static pressure inside the scroll, at a high flow-rate operating point, the fluid velocity increases from the scroll start to the scroll end, and the pressure at the scroll start is higher than that of the scroll end, so that there is practically no flow recirculation from the scroll end into the scroll start in the tongue portion 05 (joint between the scroll passage and the outlet passage).
On the other hand, at a low flow-rate operating point, the fluid velocity decreases from the scroll start to the scroll end, and the pressure at the scroll start is lower than that of the scroll end, so that there occurs flow recirculation from the scroll end into the scroll start in the tongue portion. This phenomenon causes the following pressure losses inside the scroll.
(1) First is a pressure loss caused by flow separation. The fluid flows toward the discharge port of the scroll spirally along the outer circumference of the inner wall of the scroll. The flow in the boundary layer near the wall surface is sucked into the scroll start by a pressure gradient at the flow passage joint in the tongue portion, whereby flow recirculation occurs. Flow separation occurs at this time at the flow passage joint in the tongue portion, which forms a high pressure loss region.
(2) Second is a pressure loss caused by friction. The recirculating flow that has lost energy by the separation accumulates in the center of the cross section of the scroll passage. Since such flow has low pressure, it causes to increase the pressure gradient towards the center of the scroll cross section, as a result of which the velocity of the fluid flowing spirally inside the scroll passage is increased. This increases the pressure loss by friction inside the scroll passage.
It then follows that flow recirculation that occurs in the tongue portion is the major cause of pressure loss inside the scroll at a low flow-rate operating point.
Patent Document 1 discloses a technique for improving the characteristics of the spiral flow inside the scroll passage, wherein the scroll passage is formed to have a peculiar non-circular cross-sectional shape. However, it does not disclose how to minimize the amount of recirculation near the tongue portion for better machine performance. Patent Document 2 shows reducing the negative pressure region near the tongue portion, but it relates to improvement by means of a diffuser and does not disclose improvement of the scroll cross-sectional shape for better machine performance.